The present invention relates to an on-demand ink-jet printing head that squirts ink from nozzles to form dots on recording paper. More particularly, the present invention relates to a piezoelectric ink-jet printing head that squirts ink by applying electric energy to a piezoelectric element, so that an oscillating plate is deflected to apply a pressure to a pressurizing chamber having ink stored therein, and further relates to a method of manufacturing the piezoelectric ink-jet printing head.
An ink-jet printing head using a thin-film piezoelectric element is disclosed in the specification of, e.g., U.S. Pat. No. 5,265,315.
FIG. 20 shows the cross section of the principle element of a conventional ink-jet printing head. This cross-sectional view shows the principle element of the ink-jet head printer head taken in a transverse direction of an elongated pressurizing chamber.
The principle element of the ink-jet printing head is formed by bonding together a pressuring chamber substrate 500 and a nozzle substrate 508. The pressurizing chamber substrate 500 comprises a silicon monocrystalline substrate 501 having a thickness of about 150 .mu.m. An oscillating plate film 502, a lower electrode 503, a piezoelectric film 504, and an upper electrode 505 are formed, in that order, on the silicon monocrystalline substrate 501. Pressurizing chambers 506a-506c are formed deep in the silicon monocrystalline substrate 501 in a thicknesswise direction thereof by etching. Nozzles 509a-509c are formed in the nozzle substrate 508 so as to correspond to the pressurizing chambers 506a to 506c, respectively.
The technique of manufacturing such an ink-jet printing head is disclosed in the specification of U.S. Pat. No. 5,265,315. In the steps of manufacturing the pressuring chamber substrate, a silicon monocrystalline substrate (i.e. a wafer) having a thickness of about 150 .mu.m is divided into unit areas, each of which is formed into the pressurizing chamber substrate. A flexible oscillating plate film for use in applying a pressure to the pressurizing chamber is laminated to one side of the wafer. Piezoelectric films that generate a pressure are integrally formed on the oscillating plate film so as to correspond to the pressurizing chambers by thin-film manufacturing methods such as a sputtering method or a sol-gel method. The other side of the wafer is repetitively subjected to formation of a resist mask and etching. As a result, a set of pressurizing chambers partitioned by side walls are formed. Each side wall has a width of 130 .mu.m and has the same height as the thickness of the wafer. By virtue of the above-described manufacturing method, the pressurizing chambers 506a to 506c, each of which has a width of 170 .mu.m, are formed. For example, in a conventional ink-jet printing head, a row of nozzles 509, each of which has a resolution of about 90 dpi (dot/inch), are directed to the recording paper at an angle of 33.7 degrees, thereby achieving a print recording density of 300 dpi.
FIG. 21 is a schematic representation of the operating principle of the conventional ink-jet printing head. This representation shows the electrical connections of the principle element of the ink-jet printing head shown in FIG. 20. One electrode of a drive voltage source 513 is connected to the lower electrode 503 of the ink-jet printing head through an electrical wiring 514. The other electrode of the drive voltage source 513 is connected to the upper electrode 505 that correspond to the pressurizing chambers 506a to 506c through an electrical wiring 515 and switches 516a to 516c.
In the drawing, only the switch 516b of the pressurizing chamber 506b is closed, and the other switches 516a and 516c are open. The pressurizing chamber 506c having the switch 516 opened is waiting to squirt ink. The switch 516a is closed at the time of a squirting operation (see 516b). A voltage is applied to polarize the piezoelectric film 504 in the direction as designated by A. In other words, a voltage which is the same as the voltage applied to cause polarization in polarity is applied. Then, the piezoelectric film 504 expands in its thicknesswise direction, as well as contracting in the direction perpendicular to the thicknesswise direction. As a result of the expansion and contraction of the piezoelectric film, a shearing stress acts on the boundary between the piezoelectric film 504 and the oscillating plate film 502, so that the oscillating plate film 502 and the piezoelectric film 504 deflect downwardly in the drawing. As a result of the deflection, the volume of the pressurizing chamber 506b is reduced, so that an ink droplet 512 is squirted from the nozzle 509b. If the switch 516 is opened again (see 516a), the deflected oscillating plate film 502 will be restored to its original state, thereby expanding the volume of the pressurizing chamber. Consequently, the pressurizing chamber 506a is filled with ink through an unillustrated ink supply channel.
However, the following problems are encountered in improving the print recording density with use of the structure of the example of the conventional ink-jet printing head.
First, it was difficult to improve recording density. A demand for high-resolution printing is increasing day by day with respect to an ink-jet printer. To respond to this demand, it is inevitable to increase the density of nozzles by reducing the quantity of ink to be squirted from one nozzle of the ink-jet printing head. If the nozzles are tilted in the direction of scanning, the print density will be further improved. The pressurizing chambers and the nozzles are arranged on the same pitches, and hence it is necessary to increase the density of the pressurizing chambers, i.e., it is necessary to integrate the pressurizing chambers, in order to realize high-resolution printing. For example, in the case of an ink-jet printing head having a resolution of 180 dpi, it is necessary to array the pressurizing chambers on a pitch of about 140 .mu.m. More specifically, as a result of optimizing calculation of an ink squirting pressure and the amount of ink to be squirted, a pressuring chamber having a width of about 100 .mu.m and a side wall of the pressurizing chamber having a thickness of about 40 .mu.m are ideal.
There are structural limitations on the side wall of the pressurizing chamber. Specifically, if the side wall is too high compared to its width, the rigidity of the side wall will become insufficient when a pressure is applied to one pressurizing chamber. If the rigidity of the side wall becomes insufficient, the side wall deflects, which in turn causes an adjacent pressurizing chamber, originally supposed not to squirt ink, to squirt ink (this phenomenon will hereinafter be referred to as "crosstalk"). For example, if a pressure is applied to the pressurizing chamber 506b, as shown in FIG. 21, the side walls deflect in the direction designated by B because of deficiency of rigidity of the side walls 507a and 507b. In turn, the pressure of the pressurizing chambers 506a and 506c also increase, and therefore the nozzles 509a and 509c also squirt ink. The thickness of the side wall becomes smaller as the resolution of the ink-jet printing head increases, as a result of which the above-described phenomenon becomes more noticeable.
It is only necessary to increase the thickness of the side wall in order to prevent the crosstalk phenomenon. However, it is impossible to excessively increase the thickness of the side wall in order to respond to the demand for improved resolution of the ink-jet printing head.
In contrast, it is also possible to prevent the crosstalk phenomenon by reducing the height of the side wall compared to its thickness. However, in order to safely handle the wafer during the manufacturing step, the wafer is required to possess sufficient mechanical strength. Therefore, the wafer must have a predetermined thickness. For example, in the case of a silicon substrate having a diameter of 4 inches .phi., a resultant wafer will deflect or will become very difficult to handle during the manufacturing step if the thickness of the wafer is reduced to becomes less than 150 .mu.m.
For these reasons, it was difficult to prevent the crosstalk while improving a resolution as well as ensuring the rigidity of the side wall.
Second, it was difficult to manufacture an inexpensive ink-jet printing head from the industrial viewpoint. To reduce the piece rate of the ink-jet printing head, all that needs to be done is to increase the number of pressurizing chamber substrates which can be formed at one time by increasing the area of the wafer (to e.g., a diameter of 6 or 8 inches .phi.). However, as previously described, it is necessary to increase the thickness of the wafer in order to ensure its required mechanical strength as the area of the wafer increases. If the thickness of the wafer increases, it becomes impossible to prevent the crosstalk, as having been previously described.